Automatic telephone switching system



Sept. 3, 1963 1 T..ANDERsoN ET AL 3,102,930

AUTOMATIC TELEPHONE SWITCHING SYSTEM 8 Sheets-Sheet l Filed March l5,1959 A TTORNEV L. T. ANDERSON x-:T AL 3,102,930 AUTOMATIC TELEPHONESWITCHING SYSTEM 8 Sheets-Sheet 2 Sept. 3, 1963 Filed March l5, 1959 ATTORNE V Sept. 3, 1963 L. T. ANDERSON x-:T Al. 3,102,930

AUTOMATIC TELEPHONE SWITCHING SYSTEM Filed March 13, 1959 8 Sheets-Sheet5 FIIIIIIL ATTORNEY Sept. 3, 1963 1 T. ANDERSON ET AL 3,102,930

AUTOMATIC TELEPHONE SWTTCHING SYSTEM 8 Sheets-Sheet 4 Filed March l5,1959 wlw ATTORNEY DIG/T REGISTRA T/ON Sept. 3, 1963 Filed March 13, 1959L. T. ANDERSON EVAL AUTOMATIC TELEPHONE SWITCHING SYSTEM 8 Sheets-Sheet5 TO FIG. 4

/NI/ENTOPS L. T ANDERSON E'. G. CRANE, JR.

ATTORNEY DIG/7' REGISTRATION Sept. 3, 1963 L. T. ANDERSON ET AL3,102,930

AUTOMATIC TELEPHONE SWITCHING SYSTEM Filed March 13, 1959 8 Sheets-Sheet6 /Nl/ENTO/ps L- 7: SE. HGQQQMCLM ATTORNEY E. G. CRANE, JR. K

Sept. 3, 1963 L. T. ANDERSON ETAL 3,102,930

AUTOMATIC TELEPHONE swTTcHING SYSTEM Filed March 13, 1959 8 Sheets-Sheet7 Sept. 3, 1963 L. T. ANDERSON ETAL 3,102,930

AUTOMATIC TELEPHONE swTTcHING SYSTEM 8 Sheets-Sheet 8 Filed March 15,1959 YVA dll

QVA N Ww @L EN MA w R AC w TG. uw. LE E s S p WWMMQ m W EQ N w w m ATTOPNEV United States Patent O Summit, NJ., assignors to Bell TelephoneLaboratories, Incorporated, New York, N.Y., a `corp'oration of New YorkFiled Mar. 13, 1959, Ser. No. 799,260 v 14 Claims. (Cl. 179-426) Tlhisinvention relates to telephone switching systems 'and more particularlyto `a telephone central ollice in which private branch exchangesubscribers and community subscribers share common facilities. Communitysubscribers in this context encompasses various types rolt subscribersother than private branch exchange subscribers including flat rate,private and party lines, coin lines, etc.

In the past it has been customary to supply private branch exchanges asintegral units to customers who required sucb service. The privatebranch exchange, or PBX, would generally Ibe installed at the locationof the customers premises.

Y The extensions or stations served by thePBX are usually terminated atthe PBX switchboard lor switching tacilities. From the private branchexchange a group `of trunks considerably smaller in number than thenumber of disadvantages inbere in the use of conventional privateAlthough completely operative and useful, as evidenced by the widespreadacceptance of PBX service, a number tot disadvantages inherein the useofaconventional private branch exchange arrangements.

Thus, in order 'for a subscriber outside of a PBX to call ran extensionwithin rthe PBX, it is generally necessary to dial the directory numberof the private branch exchange and thereafter be connected manually tothe desired extension. As an alternative meas-ure, in-dialing:facilities have been made available `to a limited extent but these areattended by a considerable magnioation in complexity of the switchingarrangements."

An additional discommodious feature in using PBX facilities stems from anumber of technical obstacles that have heretottore slowed thedevelopment of certain aspects of PBX facilities behind the rapid4advance of general switching technology. This situation is seen mostdramatically in area where `direct distance dialing is available, i.e.,where ia subscriber may dial a nationwide long distance call withoutoperator intervention. `One `of the barriers which prevents theapplication of direct distance dialing to private branch exchangeextensions is rooted in the problem of individually billing theextension 4for [the appropriate toll charges. Since each extension isterminated at the private branch exchange and must share cmmon trunkingfacilities to reach the telephone central loice, automaticidentification of the extensions at the central oilice has been anonerous and `generally insurmountable obstacle.

The usual, and from the standpoint of efficiency and economy,unsatisfactory, recourse is to burden the operators at privatebranchexchanges with the dutyof identitying and manually attributing tollcharges for Hong distance calls to the individual extensions lfrom whichthe calls have been placed.

It is therefore an object of this invention to automatically attributetoll lor revenue charges for long distance and other `calls directly tothe PBX extension concerned.

It is .a further objectof this invention to enable yautomaticidentiication of each PBX extension from the central o'ice.

In consequence of the universally increasing use of PBXs, a continuallygrowing number of calls rare being eilected between one extension of aPBX and another extension o' a different PBX, ie., inter-PBX calls. 'Incom pleting such ra call, it is necessary ttor the party originatingPatented Sept. 3, 1963 2 .i the call to establish access -to outgoingtrunk to a` central lotlice beiiore he can dial tbe directory number otthe called PBX. Numerous automatic and semiautomatic facilities areavailable rfor enabling the calling party to seize a trunk to thecentral oice. i

Wben the central office receives information relating to the calleddirectory number, it establishes a connection over a trunk to Ithecalled PBX. Unless the highly special-` `It is therefore another objectof this invention to permit automatic interconnection between extensionsott `different PBXs by dialing thecalled PBX extension directly andwithout operator intervention.

Perhaps the most signiiicant ineiliciency inhering in the use of #largePBX facilities is the fact that, in effect, it provides -a duplicationof a large part of the attached centralgotice facilities. A SOOO-linePBX, for example, is an install-ation comparable in magnitudeandcomplexity to a moderate sized telephone ,central ollice.` It wouldrappear manifestly inetlicient to throttle the .outgoing trailic flowfrom such :a large telephone installation by extending a limited numberof lines to a conventional telephone central oce, over which linesindividual subscribers at the PBX extensions are forced vto route theiroutside calls. This, however, has been `a necessary practice in thepast.` f The situation is renderedparticularly acute in Van environmentin which a relatively large PBX services an industry or otherestablishment located 4in a moderate sized or relatively smallcommunity. Here tbe duplication land ineiectiveness are shown insharpest relief. fA complete, independent, switching installation mustbe provided for the'private branch exchange. In addition, separate andvirtually identical switching tacilities must be provided for theindividual subscribers in the community. In order for subscribers in thecommunity to call PBX extensions and vice versa, it is necessary toproceed tortuously through paths including the limited number of linesbetween the PBX Iand the central otiice `adverted to above.

This diculty is further compounded by the tact that the PBX may" belocated in a community which is` too small to bear the expense attendingthe installation of a modern automatic switching system. Often a complexand extensive PBX installation having the mostcomprehens'ivefacilities'm'ay be required to serviceV avery large industrial entity ina location proximate to a` small suburban or semi'rural community. Inconsequence, intra-PBX service facilities may be of the highest qualitybutin order for a` `private branch exchange `subscriber who originatesbis call `from aPBX extension tru-communicate dated facilities of thecommunity central office.`

l To some extent this forces an involuntary degradation of the qualityof service availableto the PBX subscribers. By 'compelling the PBXsubscribers to route their outside connections through the comparativelymodest facilities available in the central oice, the PBX subscribersare, in effect, limited to the same quality of service available to theresidents of the town. j i

It is therefore an object of this invention to provide for theconnection of PBX subscribers to points in the telephone network outsideof the PBX without passing through a community telephone centralofi'lce.

It is another object of this invention to provide for automaticconnection to a PBX extension by a subscriber Y extensions in a `largeAPBX installation and community lines in a community located adjacentlto the PBX to share commonv central oliice facilities. p

Still another object of this invention is to provideva 'p combined PBXandtelephone `central otlice wherein the switching `facilities providePBX service for PBX subsoribersl and regular telephone rser-vice forcommunity subscribers.

lIt is a further object of this'invention'to permit economies'in theinstallation and operation of a telephone plant by adapting thetelephone equipment to perform dual service functions.

These and other objects of the invention are realized It is anotherobject of this invention to permit PBX in a telephone switching centraloffice, for example a No. '5 crossbar office of the type described inPatent No. 585,904 of A. J. Busch on February 19, 1952, wherein theequipment is modified to provide PBX facilities for PBX Vsubscribers andconventional telephone facilities for `other subscribers. Y

As `part of this arrangement telephone subscribers in the community havelines which are terminated in the usual manner on verticals of crossbarswitches. Each private community telephone customer has an individualtermination at the telephone central office whereas party linesubscribers aresupplied with common terminating facilities in awell-known manner.

However a significant departure `from routine practice is focused 'inthe'disposition of the PBX arrangement. In lieu' 'of 'the usualPBXfacilities `which 4are physically locatedion or proximate to the PBXsubscribers premises, all `of the PBX facilities are concentrated in thesame telephone .central office to which the private line subscriberslare connected. A separate line is carried `from eachfPBX extension to'-a combined PBX'and centraloflice where aV termination lis individuallymade for each PBX 'extension on a vertical .ofa crossbar switch in amanner analogous to that utilized for termination of private linesubscribers. Thus PBX extensions and private lines are both vterminatedin .the same telephone central oice.

The private lines in the'community may dial -conventionally to `effecttelephone connections `and will be servi iced V-by the telephone centraloice in la manner similar to that of lother telephone central oce's. Porexample,

n ta private line subscriber vseeking to effect a connection to anothersubscriber may dial a seven-digit code in which thetirst threel digitsare the otice code and the last four identify the subscribers Vdirectorynumber. If the telephone central oi'ce includes facilities for directdistance dialing,V i. e., xlong' `distance dialing without operatorassistance, a ten-digit code may advantageously be utilized. In

` this instance the -rst three digits comprise a nationwide area or.regionalcode and the last seven indicate the telephone exchange andsubscriber directory number.

viIn the case of theJPBX extensions, however, the telephone centraloiceprovides .a totally'` different service facility. Insofaryas the kPBXextensions are concerned the telephone central oliice is a complete andintegrated PBX'."For example, in dialing another PBX extension from iagiven PBX extension,` the calling party need only dial the digits of thecalled extension without incorporating any telephone oice code. Thecombined central oce PBX recognizes the call Ias ian intra-.PBX call and4 elfects a connection to the called PBX extension through the sharedfacilitiesV of the central olice. Thus in a PBX arrangement whichincludes less than 7000 lines only four digits need be ydialed to reachany PBX extension from another VPJBX extension.

The flexibility that inheres in the instant invention is demonstrated bythe fact that PBX extensions which seek to etect connections to callsoutside the PBX are handled by the vtelephone central oice in a mannerV.which resembles' thev treatment 4for private line subscribers; Thusthe quality of service to an individual PBX extension is enhanced. Sinceeach PBX extension is individually terminated at thecombined PBX andcentral otice, automatic identification of the PBX extension may beelected in a manner similar to that for the other private or cornmunitylines connected to the central office. In'this manner the chief obstacleto direct distance dialing :by PBX extensions is overcome.

In an illustrative embodiment of the invention a PBX subscriber seekingto make an intra-PBX call need only dial four digits. To make callsoutside the PBX an initial directing code consisting of a single digitis dialed. Thereafter the PBX subscriber at an extension will dial aseven or ten-digit number consisting of an oice code and directorynumber or area code plus office code. and directory number' in a mannersimilar to that of the community subscribers.

For calls to the PBX from outside lines where the ca1ling party is notaware of the proper extension number, a conventional PBX switchboard `isemployed. An'operator situated atthe switchboard renders assistance insuch incoming call situations. It is evident yfrom the configuration ofthe combined PB X and telephone central oce that no trunlcing facilitiesare required -between the PBX and telephone central otiice .as inconventional arrangements since eachextension is, in effect,individually terminated at the central o'ce.

A feature of this invention includes separate originating registerfacilities for PBX extensions and community lines. f

Still another feature of this invention includes preregister apparatus-in said second type of register adapted to store an initial directingcode.

Still another feature of this invention comprises arrangements in saidpreregister for inhibiting the registration of predetermined digits.

Another feature includes equipment in `said second register .forregistering digits of a lower order than said predetermined digits. i vY Still vanother feature comprises arrangements in said second registerfor registering digits of a higher order than said predetermined digits.:Y

A further lfeature includes equipment in -said second registerresponsive to the reception of la predetermined number of digitsv andthe expiration of a predetermined period of time Vfor disenabling saidregister.V y Y Still another feature of this invention includesfacilities i for dialing long distance or toll calls -by said PBXextensions after registration-of a directing code. l

A further .feature of this invention includes apparatus forautomatically identifying and billing PBX extensions ontoll calls.

Stillanother feature of this invention includes equipment for.recognizing intra-PBX calls and completing said calls on the receptionof a lesser number of digits than `required to interconnect communitylines.

' Still another feature includes apparatus for direct indialing to PBXextensions from community lines and from remote central offices.

It is apparent from the arrangements incorporated in the prent inventionthat vthe disadvantage ofduplication inherent in preexistingarrangements wherein a large and costly PBX facility is locatedproximate to a small community is surmounted. Thus in a PBX facilitywhich includes'illustratively 6000 extensions located near a communityof, for example, 1500 lines, a combined telephone central office and PBXapparatus will provide dual service features for both.

yThe community benefits materially from the enhancement of the qualityof service available to it since although a relatively small number ofcommunity lines are involved, a modern telephone plant is economicallyjustitiable.

The PBX subscribers benefit directly from the lack of necessity toproceed through common trunking facilities to the telephone centraloffice. In addition, the quality of service to PBX extensions ismarkedly improved since direct distance dialing is now available to eachextension. Thus both the PBX subscriber and the community share mutuallyin the benefits occasioned by the utilization of the instant invention.

These and other objects and features of the invention may be morereadily comprehended from an examination of the following specificationand attached drawing, in which:

FIG. 1A is a block diagram of apparatus for extending dialingconnections from community and PBX subscribers to the receptiveoriginating registers;

PIG. 1B indicates in block diagram form the equipment included inimplementing an intraoiiice connection between a PBX subscriber and acommunity or PBX subscrber in the combined PBX and telephone centraloice;

FIG. 1C illustrates in block diagram form apparatus in the combined PBXand telephone central oice utilized in completing an outgoing call to aremote PBX or central oiice which is originated by a calling PBX orcommunity subscriber;

FIGS. 24 represent in detached contact form portions of the PBXoriginating register shown in dotted outline in FIGS. 1A, 1B and 1C;

FIG. 5 `graphically depicts the functional relationship of the linerelay L and the counting relays 2P1-2P5 of FIG. 2; and

FIG. 6 is a sequence chart indicating the time sequence of `operation ofthe relays included in FIGS. 2-14.

GENERAL DESCRIPTION Dialing Connection t Referring now to FIG. 1A, anillustrative embodiment of the present invention is shown in blockdiagram form. The apparatus outlined represents a portion of a No. 5crossbar telephone oflicc of the type disclosed in detail in theabove-referred to patent of A. I. Busch. The equipment therein describedis modified in accordance with the present invention in severalparticulars described in detail herein.

In FIG. 1A the equipment shown in solid outline form representsconventional apparatus described in the Busch patent and the PBXoriginating register shown in dotted outline represents modifiedapparatus incorporated by the present invention. The detailedmodifications required in the originating register shownin dottedoutline form in FIG. 1A are disclosed in detail inFIGS. 2-4.

In PIG. 1A a community or private customer 1 is connected to a line linkframe 2 in a manner similar to that described in the Busch patent. A PBXsubscriber 3 is connected to a line link frame 4 in a similar manner.For purposes of clarity the PBX subscriber is shown connected to anindependent line link frame although it is consonant with the practiceof the present invention to connect both community subscribers and PBXsubscribers to the same line link frame.

When la community subscriber lifts the receiver from the switchhook, aline relay is operated which causes the line link frame to inform the.line link marker connector that a marker is required. -An idle markeris selec-ted by the line link marker connector and the identity of thecalling =1ine is transmitted to the marker. The marker determines theclass of service and equipment location of the register markerconnector.

calling line and establ-ishes if an idle originating register is`available and the number of the |trunk ilink iframe on which the idleregister appears.

By virtue `of a class of service mark delivered to the marker, whichdifferentiates between community subscribers and PBX subscribers in awell-known manner similar to the distinction between coin stations, andhat rate stations, etc., the marker invariably selects a communityoriginating register 6 to receive the dialing signals of a communitysubscriber.

By the same token when `a PBX subscriber initiates a call, a similarprocedure is undertaken by the marker which :always selects on a classof service mark basis a modified or PBX originating register to receive`the dial signals from the PBX substation. FIG. 1A manifestly indicatesthat separate originating registers are employed for the communitysubscribers and `for the PBX subscribers. However, as is well known inthe art, the marker performs its services on a time division basis andthe marker is connected to only one of the originating registers, linelink iframes and trunk link frames, etc. at one :given instant.

To continue the implementation of the dialing connection the marker thenselects `an idle channel between the community subscriber 1 and thecommunity originating register 5. `Information concerning the identityof the calling line, equipment number, `class of service, etc. arestored in the originating re-gister by the marker 'which now releases.

The community originating register now furnishes dial tone to thecommunity subscriber and is prepared to receive the dialed digits. Whendialing .is completed the originating register seizes the marker andtransmits the registration to it. This information now includesequipment location, class of service of the calling subscriber, numberof the line link frame and the called direc-tory number.

If the call in progress is originated by a PBX customer, the marker isconnectable to the line link Vframe 4 and to the PBX originatingregister `as indicated by the dotted arrows to perform Ifunctionssimilar to those described above.

Intraoie Call from PBX Customer to Community Subscriber Referring now toFIG. 1B, when the PBX subscriber lifts his receiver from the switchhook,a dialing connection to the P-BX originating register is established asdescribed above.

Since the assumed destination of the `call is outside fthe PBX, theoriginating party must dial a predetermined directing code whichillustratively may be the digit `9. In consequence of the dialing of thedirectingA code, the PBX originating register adapts its circuitry torecognize that the call is intended for outside the PBX. Subsequentdigits following the directing code are stored in the originatingregister digit storageunits. To reach a community subscriber in thecombined PBX and central oce the calling PBX subscriber mayillustratively dial the directing.

code 9 followed by the seven digits lrepresenting the office exchangecode and the called directory number.

The equipment in the PBX originating register adapted to recognize andstore the directing code includes arrangements (detailed herein) fordynamically yascertaining during the dialing of the first digit Whetherthe first dialed digit is lgreater or less than the directing codenumber.

After the dialing of the directing code the PBX customer dials thedirectory number of the called community subscriber (assuming this is anintraoice call) and the directory number is stored in the originatingregister. After the called directory number has been dialed the PBXoriginating register engages a marker through the originating Theregister then transmits the line equipment location of `the calling lineand the digits of the called number to the marker. The markerestablishes that the called number is in the same office as the 7calling 'line' and establishes an intraotiice connection therebetween. Aterminating connection is set up between the called `line and theintraofrice trunk and an originating connection is set up between thecalling lline and the intraoce trunk.

`In establishing the terminating connection the marker obtain-sinformation from the number group concerning the equipment llocation ofthe called number and the type of ringing.

IAfter the marker connects the ringing selection switch it releasesitself and the originating register from the intraoflice connection. Thesubscribers are interconnected and the trunk controls ringing andsupervision of the call.

Y Intra-PBX Coll Between Two PBX Customers l -In this type of call thePBX originating register is connected to the calling PBX subscriber inthe manner described above but the PBX customer does not dial theprea'ssigneddirecting code. Instead the callin-g customer proceedsdirectly to dial the digits of the PBX extension which mayillustratively include four digits. Since the rst dialed digit is notthe `preassigned directing code the originatin-g register determinesthat an intra-PBX Ycall is in progress and engages a marker immediately`following the dialing of the fourth digit representing the PBXextension number. The PBX originating register than transmitsinformation concerning the calling :line and the digits of the Outgoing(Interofce) Call By PBX Customer Referring to FIG.`1C, when a PBXsubscriber removes the receiver from the switchhook, a dialingconnection is established to a modified or PBX originating register inthe manner described above. To complete a call to the remote yotiice theoriginating PBX caller dials the directing code 9. The originatingregister recognizes that this is a call having .a destination outsidethe PBX and pre-pares to receive additional dialed digits beginning witha regional or'o'ice code. When the subscriber has dialed the remainingdigits of the called number the originatingregister enga-ges a markervthrough the originating register marker connector. ,'Ilhe registertransmits the equipment location and class of service of the callingsubscriber, the number of the line link frame used in the dialingconnection and the'called number to the marker. IThe marker translatesthe cnice codeto determine that thecall is to be completed to a remoteoffice `,and requires an outgoing sender.

yThe marker also establishes what type of pulsing to utilize incommunicating with the connecting ofice, i.e., multifrequency, dial,revertive, etc.v

Through an outgoing sender connector, the marker gains access to anoutgoing sender that sends the type of pulses Ywhich the connectingottic'e receives. When the outgoing sender connector is engaged themarker gains access to an idle trunk link frame and an outgoing. trunkon the iframe. the sender and completes a connection between theoutgoing trunk and the outgoing sender through the sender link. v v

` When the trunk link frame is seized the marker also connects to theline Ilink trame of the calling subscriber and establishes a channelbetween the subscriber and the outgoing trunk. The marker releases uponcompletion of its 'functions and the sender transmits the called numberto the connecting ofiice and disconnects itself and the The marker thentransfers the called number to I nier-PBX Calls When a PBX subscriber ina central otiice arranged in conjunction with the present inventionseeks to establish Va connection to a PBX subscriber in a different PBXwhich is also arranged lin conjunction with the present invention, adialed connection without operator intervention is sufiicient tocomplete the call. In accordance with the above-described yproceduresthe originating PBX subscriber will be connected to a PBX originatingregister and will dial the predetermined directing code 9.

The originating register recognizes the Icall as terminating outside thePBX adapts its register'stages to receive the appropriate number ofadditional dialed digits accordingly. After `dialing the directing codethe originating subscriber need dial only fthe digits representing theoce code and extension number of the PBX extension soughtto be reached.In all respects this seven-digit number is similar to the conventionalthree-digit office code `and four-digit directory number in routine use.

The marker recognizes from the called office code that the call `isintended for a remote oice and establishes an outgoing trunk connectionin the manner described in the previous section. At the called officethe [connection is completed directly to the PBX extension in a manneridentical to that for completing a call to a community line. Thusautomatic in-dialing is in effect made available between communitysubscribers who seek to reach PBX extensions and PBX customers who wishto effect calls to other PBX subscribers in other PBXs.

On incoming calls to the PBX where difliculty in effecting a 'connectionis encountered, conventional intercept techniques may be employed toroute the call to a PBX assistance operator. The PBX operator mayillustratively be connected to the combined PBX and central office in amanner similar to the other PBX lines and may be equipped with aswitchboard adapted for use in a manner analogous to that of the zerooperator in the abovereferred to rpatent of A. I. Busch.

'In addition, vwhere the PBX extension number is un- Vknown, the callingparty may dial a general published number-.for the PBX and be connectedto the PBX operator who will complete the call.

Directing CoderDz'gt Storage in Originating Register It is seen 4fromthe description heretofore that a key l[factor in the successfulcompatible operation of PBX calls and other types of calls.

"I'his'differentiation is effected through the incorporation of adirecting code digit by PBX subscribers when an outgoing call is madeand the omission'of a directing code by PBX subscribers when anintra-PBX call is undertaken. Thus, in accordance with the necessity forstoring and acting on directing code information, modifications havebeen made in those originating registers in the combined oice which aredesignated to serve only PBX customers.

These registers have modined digit storage facilities which areresponsive during the dialing of the first digit by a PBX calling partyto ascertain if, in fact, a directing code digit has been dialed. Thefirst digit if smaller than or exceeding the number of the directingcode digit should 'be stored in the iirst digit register; thus theequipnient must be of a dynamic quality in the sense that the iirstdigit register should be enabled if a digit smaller than or greater thanthe directing code digit is dialed. #Since the numerical `value of thefirst digit is not known until the entire `digit is dialed and since theoriginating register cannot ascertain Whether a number greater than thedirecting code will be dialed even if the number of pulses representingthe directing code have already been received, dynamic registerarrangements adapted to act immediately after the dialing of the numberof pulses representing a directing code followed by an interdigitalinterval have lbeen developed.

In an illustrative situation where the directing code digit is 9, theoriginating register must be arranged to permit the storage in the firstdigit storage unit of a number smaller than 9 (but exceeding 1 asexplained herein) or a number greater than 9, i.e., 10, lor 0. As willbe more fully described `herein in the detailed description, theoriginating register which serves PBX subscribers is adapted to inhibitthe storage of the digit 9 in the first digit'register. This digit isinstead stored in a special auxiliary storage unit. If the first digitdialed exceeded 9, or was `smaller than 9, the lfirst digit register ispermitted to store the digit. When the marker connects to theoriginating register after dialing has been completed, it establisheswith facility that an intra- PBX call is intended if it iinds nodirecting code digit stored and that an outgoing or other specialservice call is in effect if a digit 9 has been stored. The uniquearrangements whereby the originating register has been adapted toperform the function of storing information for both intra-PBX and`outgoing calls are made clear in the following detailed descr-iption.

Detailed Description of A pparatns-Connling Circuit yFor a comprehensivedescription 'of -apparatus suitable in conjunction with the presentinvention, reference may be made to the above-referred to patent of A.J. Busch. In the drawing and in the following description only thoseitem-s of equipment Which are essential to an understanding of thepresent invention are included. Omitted material is fully treated in theBusch patent. Reference may also be made to Patent No. 2,722,675 of l.Michal et al. issued November 1, 1955 for additional details coveringthe counting circuit.

For any digit transmitted or dialed, a train of pulses consisting of oneto ten equally spaced momentary line openings is generated by thecalling subscribers dial. The number of line openings corresponds to thenumber dialed. One for the number 1, two for the number 2, etc. and tenfor the number `0. These pulses may be at the nominal rate of l or 2Oeach second. Between these trains of pulses an interdigital intervalobtains during which the calling line loop remains closed. The pulsingrelay L, the contacts of which are shown in FIG. 2, responds to the dialpulses, and the counting circuit including `relays ZP'l-ZPS counts thenumber of pulses in each digit. During fthe interdigital interval thecount is transferred to the proper digit register unit 3A, 3F, 4G, '4L,etc. When this transfer is completed the counting relays 2P- arereleased and the counting circuit is ready to count the pulses of thenext digit. The end of fthe digit is recognized by means of a sloWrelease register advance relay (not shown), the contacts of which (FIGS.2 and 3--oontacts of relay RA) operate during the first opentpulse `andrelease during the interdigital interval. The contacts of this relaycontrol the transfer of the count foreach digit to the proper digitregister unit.

The counting and registration of a digit takes place as the contacts ofrelay L (fFIG. 2) respond to the subscriber dial and the countingcircuit counts the number of pulses. Relay L is operated each time thesubscriber loop is closed.

As will be seen in the following explanation, relay RA operates duringthe first dial open pulse When relay L releases and will remain operateduntil the interdigital interval when relay L is held' operated. The dialpulse counting circuit consists of relays ZPl-ZPS and relay ZPZA. Relays2P1 and 2PZ function as pulse dividers from relay L. The contacts ofrelays ON Iand SR are closed when the circuit is seized. When relay Lreleases on the first dial open pulse with relay SR operated, relay 2P1operates over a circuit including ground, contacts of relay L, contactsof relay SR, contacts of relay 2PZ, winding of relay ZPl to negativebattery. Relay 2P1 locks operated over contacts 210 of relay 2111 nowoperated, contacts of relay 3RA1, contacts of relay ON to ground. Whenrelay L reoperates at the end of the first. open pulse,

relay 2PZ operates over the contacts of relay L, contacts p of relayMSTl, contacts of relay 3RA1, contacts of relay ZPJ, contacts of relay2PZ, winding of relay 2PZ to negative battery.

Relay 2PZ locks to ground thnough the normally open contacts of relay2PZ, contacts 210 of relay ZPl, contacts of relay 3RA1 and the contactsof relay ON. On the next release of relay L, relay 2P1 releases inconsequence of the interruption of its holding path at the normallyclosed contacts of relay 2PZ. Relay ZPl in releasing opens 'at contacts210 the holding circuit to the ground at the contacts of relay ON forrelay 2PZ but relay 2PZ remains operated to `ground at the normallyclosed contacts of relay L over a circuit including ground, normallyclosed contacts of relay L, contacts of relay SR, contacts of relay 2PZ,winding of relay 2PZ to negative battery.

On the next operation of relay L, relay 2PZ releases. This cycle .isthen repeated with relays 2P1 and 2PZ remaining operated at the end ofeach odd numbered pulse and remaining normal at the end of each evennumbered pulse. Relays ZPS, 2P4 and ZPS are used in various combinationsto count and remember the number of operations `of relays 2P1 and 2PZ.Relay ZPS operates when relay 2P1 releases `at the start of the secondopen pulse. Relay 2134 operates when relay 2PZ operates at the end ofthe third open pulse and relay ZPS operates when relay 2PZ releases yatthe end of the sixth open pulse. The sequence of operation `of theserelays is given in part in the sequence chant shown in FIG. 6 and infull graphical form on FIG. 5.

The auxiliary counting relay 2PZA is operated at the start of the secondopen pulse when relay 2P3 operates. The function of relay ZPZA is torecognize when the counting circuit counts a first digit greater .than land causes the digit to be transferred to the A digit register 3A. RelayZPZA at contacts 310 and y311 cioses the() and 1 output leads of thecounting circuit.

Preliminary Pulses To prevent an invalid directory number from beingregisteredwhen the calling subscriber accidentally generates a singlepulse by opening the switchhook contacts before dialing, the circuit isarranged so that a l cannot fbe registered on the A digit register. Ashereinbefore stated this is the function of relay 2P2A which keeps the 0and l output leads of the counting relays open at contacts 310 and 3111until it operates as a result of the count `of a number greater than 1.

Operation of PBX Originating Register Having examined in some detail thestructural com@ position of the PBX originating register, the operationthereof will be analyzed. l

When the switchhook contacts at a PBX extension are closed inconjunction with the lifting of the receiver from conventionalregistration practice `takes place bellv pulse.

When relay L releases at the beginning of the ninth pulse, relay ZPloperates. Relays ZPS and ZPS have already operated as shown in FIGS. and6'. When relay L operates at the end of the ninth pulse, relay 2PZ Y`operates and rel-ay ZDD9 is operated (see FIG. 6).

Relay 2DD9 operates over a path including negative battery, winding ofrelay 2DD9, contacts of relay ZMAN, contacts of relay 2DD9, contacts cfrel-ay 2DD9A, contacts of -relay 4AS, contacts of relay ZPS, contacts ofrelay l2,133-, contacts of relay lZPI, contacts of relay 2PZ, contactsof relay SIRAII and contacts of relay ON to ground. Relay ZP4 yisreleased as a result of the operation of relay 2PZ in consequence odithe interruption of the holding circuit therefor at the contacts 211 ofrelay 2PZ.

Relay ZPZA releases in consequence of the operation of relay 2DD9 at thecontacts 212 of relay 2iDD9l in series therewith. The operation orfrelay ZDDS opens the operating path to the digit relays lSAQ; and 3A'7as a result of the opening of the contacts of relay ZDD9 in seriestherewith.

YDuring the interdigital interval which obtains after the transmissionof the digit 9, relay RA releases as explained above and relay 3RA1operates over a path including the contacts of relay IRA and thecontacts 312 of relay ON. Relays ZPS, 2P1, 2PZ and ZPS now release asshown in lFIG. 6. Relay 2DD9LA operates in consequence of the release ofrelay ZPS over a circuit including negative battery, winding of relayZDDQA, contacts of relay ZiDD9, contacts of relay ZPS and the contactsZ13 of relay ON to ground.

The 2 and 7 leads to the digit relays 3A2 and 3A7 are reclosed inconsequence of the closing of the contacts of relay 2DD9A in seriestherewith.

` tenth pulse, relay L reoperates and relay 2PZ is released.

Relay 2MAN is operated when relay L reoperates in view of the previousoperation of relays 2P3 and ZPS and the release of relays `2fl4 and 2131as shown in the v sequence chart of FIG. 6.` rllhe operating path forrelay ZMAN includes ground, contacts of relay ON, contacts of relay3RA1, contacts of relay ZPl, contacts of relay 2P3, contacts of relays2P4, ZPS, 4AS and winding of relay ZMAN toV negative battery.

Relays 2DD9 and 2P2A are respectively released and operated inconsequence of the operation of relay ZMAN. Relay PZA operates over apath including ground, contacts of relay ON, contacts of relay ZMAN,winding of 4relay PZA to negative battery. Relay 2DD9 releases at thecont-acts of relay ZMAN in series therewith.

At the conclusion of the tenth pulse and after the interdigiltalinterval, relay RA releases as explained above and relays 3RA1, 3A4 and3A7 operate.

Relay 3A4 operates over a path including negative battery, winding ofrelay 3A4, contacts of relay 4AS, output lead 4, contacts ott relay3RA1, contacts of relay 21P3, contacts of relay ZPS, contacts of relay2P4, contacts of relay 2PZ, contacts of relay RA, contacts of relay4MST1, contacts 3:12 of relay ON to ground. y

Relay 3-A7 operates over a path from Ynegative battery, winding of relay3A7, contacts of relay 4AS, output lead 7, contacts of relay 3RA1,contacts of relay 2DD9,

` contacts of relay 2P3, contacts of relay ZPS, contacts oaf relay RA,contacts of relay 4MST1, contacts y312 of re- `lay ON to ground.

Thus when a PBX subscriber dials an initial 0, relayV 2DD9 operates atthe end of the ninth pulse and digit registers SAZ and 3A7 aremomentarily disabled to prevent the 9 from being registered in the Adigit register. i On the tenth pulse, lhowever, relay QMAN operates andreleases relay 2DD9 at the contacts of relay l2 ZMAN in series therewithto permit storage of the initial 0l in the A digit register.

Digit Steering The digit steering circuit, a portion of which is shownin FIG. 4, connects the output leads 0, l, 2, 4 and 7 (FIG. 3) of thepulse counter to the digit registers 3A, 3F, etc. as the digits arereceived. The steering circuit is also used to indicate when dialing iscompleted. circuit consists of one relay per digit of which relays 4AS,`4BS, 4CS and 4LS are shown. The A digit steering relay 4AS 'is operatedwhen olf-normal relay ON operates at the .start of a call. Relay 4ASoperates from ground, cont-acts of relay ON, contacts of relay ZPZA,winding of relay 4AS .to negative battery. Relay 4AS locks operated overthe contacts orf relay 4AS, contacts of relay 4BS and the contacts ofthe remaining steering relays to the contacts of relay ON and ground.

The advance of the digit steering relays is under the control of relay3RA1. When relay BRAII -operates at the end of the iirst digit withrelay ZPZA operated as explained above, relay 4BS operates over a pathlncluding negative battery, Winding of relay 4BSl, contacts of relay4BS, contacts of relay 4AS, contacts of relay RAl, contacts of relay2PM, contacts of relay ON to ground. Relay 4BS locks operated throughthe contacts of relay 4CS and the remaining steering relays to thecontacts of relay ON and ground over the same locking path previouslydescribed for relay 4AS. When relay 3RA1 releases at the start of thenext digit, relay 4AS releases at the contacts of relay 3RA1.

'llhis action repeats for each digit with the steering relay for thenext digit operating when relay 3RA1 operates at the'end of a digit andthe steering relay for the digit just registered releasing when relayllRAll releases at the start of the next digit. rllhe tive output leads0, 1, 2, 4 and 7 of the counter are carried through individual transfercontacts 4AS1, 4AS2, etc. on the steering reays.

'Ihe counting and registration of a digit takes place as relay Lresponds to the subscriber dial and the counting circuit counts thenumber of pulses. After the last pulse gf the digit, relay L remainsoperated and relay RA reeases. Y

Relay RA closes a grolund over the contacts 3112 of relay ON and thecontacts of relay-4MST1 through two separate contacts of relay RAto thetranslating contacts of the counting relays. The groundemerges throughcontacts of relay 3RA11 on two of the-output leads 0, l, 2, 4 and 7 andis connected by the contacts of a steering relay 4AS, 4BS, etc. to aVdigit register to cause operation of two of the live digit registerrelays 3A0, 3A|1, etc. in register 3A, BF, etc.

Directing Code Digit Dialed by PBX Subscriber- Storage of Next Digit inFirst Digit Register -In accordance with the foregoing explanation it isseen that when a PBX subscriber dials an initial 9, the orginatingregister is arranged to recognize that the call is intended for adestination outside the PBX and to prepare to receive further dialeddigits which may illustratively be 7 or 10 in number.

yIt is desirable that the next succeeding digit dialed by the PBXsubscriber, which is in fact the iirst digit of the called directorynumber, be stored in the rst digit'register. To accomplish thisprocedure of storing Kthe second dialed digit in the first digitregister the steering circuit is equipped to prevent an advance when thetirst dialed digit is a 9. Referring again to tFIG. 4 it is seen thatunder ordinary circumstances when the irst dialed digit is neceived,relay BS operates at the end of the -iirst digit as explained above. Inaccordance 'with the present .modilied arrangement, however, relay BS isnot permitted to operate at the end of the rst digit since relay ZPIZAwas released in consequence of the opening of relay ZPZA in 13 view ofthe operation of relay ZDD9. As seen in FIG. 6, relay 2DD9 operates atthe end of the ninth dial pulse. The contacts `212 of relay 2DD9 inseries with relay ZPZA occasions the release thereof. ,'Ihus fwhen thefirst digit dialed is a "9, relay 4BS cannot operate in View of theinterruption of its operating path at the contacts of relay Selection ofMarker by Originating Register Referring again to FIG. 4 it is seen thatthe contacts of the steering relays are utilized to effect an energizingconnection to the marker by operating the marker start relay 4MST. Thusit is seen that if an intra-PBX is in progress and the marker should besummoned at the end of the fourth digit, the operation of relay ES, notshown, after four digits have been dialed, will result in the operationof relay y4MST over a circuit including ground, contacts of relay ES,contacts of relay 2DD9A, winding of relay 4MST to negative battery.|From what has been explained it is manifest that relay 2D-D9A does notoperate unless an initial directing code of 9 has been stored.

In consequence, if a directing code digit 9 has been dialed by a PBXsubscriber, the operation of relay ZDD9A prevents energization of relay4MST at the termination of the fourth dialed digit and instead relay4MST does not operate until in the case of a seven-digit call a total ofeight digits has ben dialed as indicated by the operation of relay HS,not shown. At the conclusion of the seventh directory number digit acircuit for the operation of relay 4MST may be traced from ground,contacts of relay HS, winding of relay 4MST to negative battery.

Thus, the dialing of an initial directing code digit 9 results in theoperation of relay 2DD9A. As seen from SFIG. 6, relay 2DD9 operates atthe end of the ninth dial pulse and relay 2DD9A awaits the expiration ofthe interdgital interval as signified by the operation of relay 3RA1 andthe release of the counting relay 2P3. If another pulse is receivedbefore the expiration of the interdigital intreval, relay 2DD9A does notoperate and relay 2DD9 eventually releases. If, however, `aninterdigital interval follows the ninth dial pulse, relay ZDDSA operatesto indicate the reception of a lirst dialed digit or directing code 9;

llt is seen that by introducing facilities in the originating registerfor discriminating between calls intended for completion within the PBXand calls intended for completion outside the PBX, the basicprerequisites for successful compatible operation of a combined PBX andcentral oice are met.` In consequence, by terminating lPBX lines in thecentral oice in the same manner that community lines are terminated,each PBX extension appears to calls originating from remote centraloices (or the same oliice) as a private line. In the same manner theconventional automatic message accounting equipment and other revenueand toll recording facilities are connectable to the individualPBXextensions in the same manner as private lines. Direct billing of PBXextensions for toll calls is thereby afforded.

By the same token automatic in-dialing to the PBX extension is availablefrom remote telephone oiices or `remote PBXs by dialing the exchangecode of the combined PBX and central oliice followed by the extensionnumber.

As to the PBX extensions, however, the fact of individual terminationfor each extension in a manner similar to that of a private line doesnot in any way impede the rusual intra-PBX facilities since theoriginating register is adapted to complete PBX calls in the usualmanner if no directing code has been dialed. Thus where a PBXillustratively includes iess than 7000 lines in the case of a four-digitnumber PBX, the PBX subscriber, by merely dialing a four-digit number,can be connected to any other PBX extension since the marker is calledin at the end of the fourth digit (iFIG. 4 as explained above).

It is understood that the above embodiments are merely exemplary andthat various modifications may be met without departing from the spiritand scope of the invention.

What is claimed is:

`l. A telephone automatic switching system including first and secondgroups of calling substations, lirst and second register means, meansfor individually connecting said first and second register means to saidfirst and second groups of stations respectively, said registers beingoperative responsive to digital signals from said sub- Vstations tostore the digital value thereof, registration inhibiting means in saidsecond register means, means responsive upon the receipt of a number ofpulses in a pulse train corresponding to a predetermined digital valuefor energizing said inhibiting means to prevent storage in said secondregister of said predetermined digital value, and means responsive upon4the receipt of at least one succeeding pulse in said salme pulse traincorresponding to a digital value higher in numerical order than saidpredetermined digital value for disabling said inhibiting means topermit storage in said second register of `said higher digital value.

2. A telephone automatic switching system in accordance with claim `lfurther comprising means in said second register operatively responsiveto said energized inhibiting means for determining that the number offurther digits to be received will exceed 'a predetermined number ofsaid digits.`

3. A telephone automatic switching system including a iirst group ofprivate branch exchange lines anda second group of community lines, saidlines including signaling means, means for terminating said first groupof lines and said second group of lines, first and second register meansaccessible and individually connectable to said iirst and second groupsof lines respectively, said registers being operative responsive todigital signals from said `lines for storing the digital value thereof,means connectable to said register means and responsive to the storageof digital signals from a calling line in one of said groups of linesrepresenting a called line in the other of said groups of lines forinterconnecting said calling and called lines, registration inhibitingmeans in said second register means, means in said second registerresponsive to the reception of a predetermined number of pulses from acalling one of said private branch exchange lines for energizing saidinhibiting means to prevent `storage in said second register of thedigital value `signied by saidpredetermined number of pulses, means in`said second register responsive to the reception of said predeterminednumber of .pulses and a succeeding pulse within a particular intervalfor disabling said inhibiting means to permit storage in said secon-dregister of a digital value exceeding that signified by saidpredetermined numer of pulses, and means in said second registerresponsive to the reception of pulses less than said predeterminednumber for disabling said inhibiting means to permit storage insaidregister of a digital value less than that signilied by saidpredetermined number.

4. A telephone automatic switching system in accordance with claim 3including in addition means in said second register responsive to thereception of signals from a calling one of said lines signifying a digithaving a value less than said predetermined number and an additionalnumber of digits representing a private branch exchange line number 'forstoring said number, and means connectable to said second register yforinterconnecting said callin-g line and said private branch exchange linerepresented by said stored number.

5. A telephone automatic switching system in accordance with claim 3including additional means in said second register responsive to thereception of signals from n a calling one of said lines signifying saidpredetermined number and lan additional number of digits representing acommunity line number for storing said line number, and meansconnectable to said second register -for interconnecting said callingline and said community line represented by said stored number.

6. A telephone central oliice including iirst and second V to store thedigital Value thereof, means in said iirst register responsive to saidsignaling means for storage of ia rst number of digits from a callin-gone of said iirst group of stations representing a particular one ofsaid second group of stations, means connectable to said iirst registermeans and responsive to the storage therein of said iirst number ofdigits for interconnecting said calling station `and `said stationrepresentted by said rst number of digits, means in said second registerresponsive to said signaling means at a calling one of said secondstations -for storing a second number of digits representing saidparticular one station, said second number of digits being smaller thansaid rst number of digits, and means connectable to said second registerand responsive to the staroge thenein of said second number of digitsfor interconnecting said calling one of said second group of stations tosaid called station. i

7. An automatic telephone central oflice including a first and second'group of substations including dia-ling means, a plurality of linellink inames, means for similarly connecting said first and secondgroups of calling substatons to said iine Ilink iframes, -rst `andsecond registers accessible to sa-.id 'line llink frames andindividually connectable to said iirst and second groups of substationsrespectively, means in said first registers responsive to che dialing bya callin'g substation in said iirst group of a rst number representing asubstation in said second `group -for storing said iirst number, me'ansconnectable to said first registers and responsive to the storage ofsaid rst number Ifor interconnecting said calling substation in saidfirst -group and said substation in said second group represented bysaid iirst number, means in said second register responsive to thedialing by a calling substation in said second sgroup or a differentnumber representing the same substation in said second group identifiedby said Ii-rst number for storing said diierent number, and meansconnectable to said second register and responsive to the storage ofsaid different number for interconnecting said ycalling substation insaid second fgroup land said substation represented by said secondnumber.

`8. A telephone vautomatic central office including a first and secondgroup of substations having dialing means, said first group ofsubstati'ons having a reliatively lower community of interest saidsecond group of substations, means for interconnecting said substationsin said second group in response to the dialling of a lirst number ofdigits from said substations, means ior inter-V connecting said'substations in said rst group in response lr6 stations in said secondgroup in response to fthe dialing of said second number of digits bysaid substations in said first group.

10. A combined telephone centriai oflice and private branch exchangeincluding a plurality of community `lines and a plurality of privatebranch exchange lines including dialing means, la plurality :of lineiink frames, means -for terminating said community ylines and saidprivate branch exchange lines on said line :link frames, first andsecond groups of registers .accessible to said yline link frames andindividually rconnectiable to Vsaid community lines and private branchexchange lines respectively, means including said first registersresponsive to the dialing of :an exchange code and directory number bysaid community ilines tfor -interconnectingsaid community lines, meansincluding said nrst registers lresponsive to the dialing of yan exchange'code and directory number by said community .lines for connecting saidcommunityV lines to said private branch exchange ylines, means includingsaid second registers responsive to the dialling of a directing code:followed by an exchange code and directory number by said privatebranch exchange "lines for connecting said private branch exchange linesto said community lines, and means in said :second register responsiveto the dialing :of only a directory number by said priviate branchexchange Ilines for interconnecting said private branch exchange lines.

y1l. A combined telephone` office l'and private branch exchangevincluding la plurality of community llines and a plurality of privatebranch exchange lines having dialing means, a plurality of \line llinkfnarnes terminating said community lines and private branch exchangeiines, Vfirs-t and 'second groups of registeins having digital storageelements and accessible to said lline link frames and individuallyconnectable to .said community and private branch exchangelinesrespectivel means including said first registers responsive to thedialing by said comm-unity iines of an exchange code follovvedby adirectory number :for Iinterconnecting said community 1li-nes :and lforconnecting 'said community lines to said lprivate branch exchange lines,means including `said second registers responsive to .the dialing bysaid private brianch exchange iines of a directing lcode [followed by anexchange code and directory number for :connecting said private branchexchange ylines to said community lines, registration inhibiting meansin said 'second register responsive to theV Vreception of saiddirecting'code for energizing said inhibiting means to prevent storageof said directing code in said digital storage element-s of said secondregister, and means in said second register responsive to the receptionof said directing code and a succeeding signal witha predeterminedinterval for disabling said inhibiting means Lto permit storage in saidregister of a digital value greater than the directing code.

l2. A combined telephone office Iand private branch exchange inlaccordance with claim lll comprising in addition means including saidsecond registers responsive to the diaiin-g by said private branchexchange ilines orf lonly a directory number for interconnecting saidprivate branch exchange lines.

13. A combined telephone oice and private branch exchange including aplurality of community lines `and a plurality of private branch exchangei-ines having dialing means, -a plurality of line link framesterminating said community lines 'and private branch exch'ange'vlines,first and second groups of registers 'having digital storage elementsand accessible to said line =link iframes, said groups of registersbeing individually conne'ctabletosaid community and private branchexchange iines respectively, means including said iirst registersresponsive to the diaiing by said community linesof 1an exchange codefollowed by la `directory number for interconnecting said communitylines and for connecting said community llines to said private branchexchange iin'es, means including said second registers responsive ytothe dialing by said private 17 branch exchange lines of a directing code.followed by an exchange code and `directory number for connecting saidprivate branch exchange lines to said community lines, additional meansincluding said second registers responsive to the ydialing 'by saidprivate lbranch exchange lines of only a directory number forinterconnecting said private branch exchange lines, registration`inhibiting means `in said second registers responsive fto mhe receptionof said directing Icode for energizing said inhibiting means to preventstoiiage of said directing code in said digital storage elements of saidsecond registers, said inhibiting means including at least a tirst andsecond relay, said rst relay being operative responsive to the receptionof a predetermined number of pulses signifying said directing code, saidsecond relay bein-g openative responsive to the operation of said firstrelay and the expiration of a time period representing an interdiigitalinterval, said first relay being `conditioned to release in response tothe reception of an additional pulse prior to the expiration of CII 18said predetermined time interval, means responsive to the `operation oflsaid second relay for blocking the storage of said directing code insaid digital storage elements, land means responsive to the release ofsaid second relay for permitting the storage in said registers of radigital v References Cited in the tile of this patent UNITED STATESPATENTS 1,747,478 Lomax Feb. 1-8, 1930 2,025,389 Hovl'and Dec. 24, 19352,883,466 Hutton Apr. 21, 1959

1. A TELEPHONE AUTOMATIC SWITCHING SYSTEM INCLUDING FIRST AND SECONDGROUPS OF CALLING SUBSTATIONS, FIRST AND SECOND REGISTER MEANS, MEANSFOR INDIVIDUALLY CONNECTING SAID FIRST AND SECOND REGISTER MEANS TO SAIDFIRST AND SECOND GROUPS OF STATIONS RESPECTIVELY, SAID REGISTERS BEINGOPERATIVE RESPONSIVE TO DIGITAL SIGNALS FROM SAID SUBSTATIONS TO STORETHE DIGITAL VALUE THEREOF, REGISTRATION INHIBITING MEANS IN SAID SECONDREGISTER MEANS, MEANS RESPONSIVE UPON THE RECEIPT OF A NUMBER OF PULSESIN A PULSE TRAIN CORRESPONDING TO A PREDETERMINED DIGITAL VALUE FORENERGIZING SAID INHIBITING MEANS TO PREVENT STORAGE IN SAID SECONDREGISTER OF SAID PREDETERMINED DIGITAL VALUE, AND MEANS RESPONSIVE UPONTHE RECEIPT OF AT LEAST ONE SUCCEEDING PULSE IN SAID SAME PULSE TRAINCORRESPONDING TO A DIGITAL VALUE HIGHER IN NUMERICAL ORDER THAN SAIDPREDETERMINED DIGITAL VALUE FOR DISABLING SAID INHIBITING MEANS TOPERMIT STORAGE IN SAID SECOND REGISTER OF SAID HIGHER DIGITAL VALUE.